NTCorrelation: Count-trefoil correlations
- class treecorr.NTCorrelation(config=None, *, logger=None, **kwargs)[source]
Bases:
BaseNZCorrelation
This class handles the calculation and storage of a 2-point count-trefoil correlation function, where a trefoil is any field with spin-3 rotational properties.
See the doc string of
Corr3
for a description of how the triangles are binned along with the attributes related to the different binning options.In addition to the attributes common to all
Corr2
subclasses, objects of this class hold the following attributes:- Attributes:
xi – The correlation function, \(\xi(r) = \langle t_R\rangle\).
xi_im – The imaginary part of \(\xi(r)\).
varxi – An estimate of the variance of \(\xi\)
cov – An estimate of the full covariance matrix.
raw_xi – The raw value of xi, uncorrected by an RT calculation. cf.
calculateXi
raw_xi_im – The raw value of xi_im, uncorrected by an RT calculation. cf.
calculateXi
raw_varxi – The raw value of varxi, uncorrected by an RT calculation. cf.
calculateXi
Note
The default method for estimating the variance and covariance attributes (
varxi
, andcov
) is ‘shot’, which only includes the shape noise propagated into the final correlation. This does not include sample variance, so it is always an underestimate of the actual variance. To get better estimates, you need to setvar_method
to something else and use patches in the input catalog(s). cf. Covariance Estimates.The typical usage pattern is as follows:
>>> nt = treecorr.NTCorrelation(config) >>> nt.process(cat1,cat2) # Compute the cross-correlation. >>> nt.write(file_name) # Write out to a file. >>> xi = nt.xi # Or access the correlation function directly.
- Parameters:
config (dict) – A configuration dict that can be used to pass in kwargs if desired. This dict is allowed to have addition entries besides those listed in
Corr2
, which are ignored here. (default: None)logger – If desired, a logger object for logging. (default: None, in which case one will be built according to the config dict’s verbose level.)
- Keyword Arguments:
**kwargs – See the documentation for
Corr2
for the list of allowed keyword arguments, which may be passed either directly or in the config dict.
- calculateXi(*, rt=None)[source]
Calculate the correlation function possibly given another correlation function that uses random points for the foreground objects.
If rt is None, the simple correlation function \(\langle t_R\rangle\) is returned.
If rt is not None, then a compensated calculation is done: \(\langle t_R\rangle = (DT - RT)\), where DT represents the mean radial trefoil field around the data points and RT represents the mean radial trefoil field around random points.
After calling this function, the attributes
xi
,xi_im
,varxi
, andcov
will correspond to the compensated values (if rt is provided). The raw, uncompensated values are available asrawxi
,raw_xi_im
, andraw_varxi
.- Parameters:
rt (NTCorrelation) – The cross-correlation using random locations as the lenses (RT), if desired. (default: None)
- Returns:
Tuple containing
xi = array of the real part of \(\xi(R)\)
xi_im = array of the imaginary part of \(\xi(R)\)
varxi = array of the variance estimates of the above values
- finalize(vart)[source]
Finalize the calculation of the correlation function.
The
Corr2.process_cross
command accumulates values in each bin, so it can be called multiple times if appropriate. Afterwards, this command finishes the calculation by dividing each column by the total weight.- Parameters:
vart (float) – The variance per component of the trefoil field.
- write(file_name, *, rt=None, file_type=None, precision=None, write_patch_results=False, write_cov=False)[source]
Write the correlation function to the file, file_name.
If rt is None, the simple correlation function \(\langle t_R\rangle\) is used.
If rt is not None, then a compensated calculation is done: \(\langle t_R\rangle = (DT - RT)\), where DT represents the mean trefoil field around the data points and RT represents the mean trefoil field around random points.
The output file will include the following columns:
Column
Description
r_nom
The nominal center of the bin in r
meanr
The mean value \(\langle r \rangle\) of pairs that fell into each bin
meanlogr
The mean value \(\langle \log(r) \rangle\) of pairs that fell into each bin
tR
The mean real part of the trefoil field relative to the center points.
tR_im
The mean imaginary part of the trefoil field relative to the center points.
sigma
The sqrt of the variance estimate of either of these
weight
The total weight contributing to each bin
npairs
The total number of pairs in each bin
If
sep_units
was given at construction, then the distances will all be in these units. Otherwise, they will be in either the same units as x,y,z (for flat or 3d coordinates) or radians (for spherical coordinates).- Parameters:
file_name (str) – The name of the file to write to.
rt (NTCorrelation) – The cross-correlation using random locations as the lenses (RT), if desired. (default: None)
file_type (str) – The type of file to write (‘ASCII’ or ‘FITS’). (default: determine the type automatically from the extension of file_name.)
precision (int) – For ASCII output catalogs, the desired precision. (default: 4; this value can also be given in the constructor in the config dict.)
write_patch_results (bool) – Whether to write the patch-based results as well. (default: False)
write_cov (bool) – Whether to write the covariance matrix as well. (default: False)